Influence of Surface Processes on Strain Localization and Seismic Activity in the Longmen Shan Fold-and-Thrust Belt: Insights From Discrete-Element Modeling

Abstract

We investigated interactions between structural deformation and surface processes in the Longmen Shan fold-and-thrust belt and the adjacent western Sichuan foreland basin (WSFB) in the eastern margin of the Tibetan Plateau. The discrete-element modeling (DEM) method was used to study the influences of the various mechanical properties of the detachments and syn-tectonic erosion/deposition on the structural evolution of the Longmen Shan and WSFB. DEM simulations demonstrated two stages of surface processes during the Late Cenozoic profoundly influenced thrusting sequences and strain localization in the hinterland and foreland portions of the Longmen Shan. Models indicate that the fold-and-thrust belt lacking surface processes propagate in a forward-breaking manner, whereas those with surface processes develop following an out-of-sequence thrusting pattern. We infer that large-scale erosion propagation from the Sichuan Basin westward to the Tibetan Plateau since Late Cenozoic caused the Longmen Shan hinterland to reach a subcritical wedge state. Tectonic activity retreats to the edge of Plateau, enhancing the rapid uplift of the Longmen Shan and inhibiting the propagation of substantial shortening deformation to the foreland basin. The foreland thrust belt slides stably along the shallow detachment, causing the initiation and growth of the Longquan fault in the leading front. These results explain why both the Longmen Shan hinterland and the western Sichuan foreland thrust belts are currently in a state of simultaneous seismic activity. Our findings offer important implications regarding the seismic potentials of other fold-and-thrust belts that interact with dynamic surface processes.